1. Field of the Invention
The present invention relates in general to the detection and monitoring of dangerous vapors and gases, and, in particular, to a tape for detecting the presence of hydride gases and for optically determining the concentration of hydride gases.
2. Description of Related Art
The semiconductor industry uses a number of hydride gases, particularly arsine, germane, silane, phosphine, and diborane, in the manufacture of semiconductor devices. These hydride gases are either highly toxic or highly flammable, so that the detection of even small concentrations of these gases is crucial. Moreover, industry guidelines recommend threshold limit values (TLVs) for each of these gases which represent the maximum time-weighted average concentration a worker should be exposed to in an eight hour period. As these gases are colorless, non-irritating, and have only a mild odor, various systems for their detection have been proposed.
One approach has been to provide a detection tape that changes color, i.e., forms a stain, in the presence of hydride gases. The intensity of the stain, which is related to the concentration of hydride gas to which the tape has been exposed, is typically measured optically, such as by using a photocell to measure the intensity of light reflected from the stain. A number of different formulations for such hydride detection tapes have been proposed.
U.S. Pat. No. 4,420,567, which is incorporated herein by reference, discloses a hydride detection tape and a tape reader that continuously monitors the hydride gas concentration by measuring the intensity of the stain formed on the detection tape. The disclosed detection tape comprises a porous tape, made of cellulose, incorporating an adsorbent material therein, such as silicic acid, magnesium silicate, magnesium oxide, or aluminum oxide, and impregnated with a processing solution consisting essentially of: (1) silver nitrate, present in an amount of 0.5% to 2.0% by weight; (2) an acid, preferably nitric acid, present in an amount of 0.1 to 5.0% by volume; (3) a glycol, such as ethylene glycol, propylene glycol, trimethylene glycol, or glycerol, present in an amount of 5 to 20% by volume, and (4) an alcohol, such as methanol, ethanol, or isopropanol.
As was disclosed in that patent, it is known that silver nitrate will react with certain metal hydrides to form a characteristic black or dark brown color. However, a detection tape simply coated with silver nitrate is extremely light sensitive and unstable, turning brown within a 24 hour period, even when sealed in a black, lightproof container. The hydride detection tape formulation disclosed in that patent has the advantage of being both sensitive to hydride gases and resistant to light. It is believed that the acid used in the solution is responsible for rendering the silver nitrate resistant to light.
The tape reader disclosed in that patent operates by moving the detection tape past an optical reader that measures the stain intensity by using a photocell to measure the intensity of the light reflected from the tape. The reader also provides outputs indicative of the measured hydride concentration and of the time-weighted average hydride concentration in a given time period.
It has been found that the nitric acid used in the processing solution of this hydride tape gradually weakens the tape over time, apparently by reacting with the cellulose, thereby limiting the tape's useful life to four to six weeks. The weakening has also been found to be more rapid with elevated temperatures. Eventually the tape may become too weak to work in the tape reader. For example, some tape readers are not able to advance tape if it has a tensile strength below about a 1 kg. for most devices.
Patent application WO 90/13025 claims that using p-toluenesulfonic acid instead of nitric acid can increase the mechanical strength of the detection tape. In particular, this reference discloses a hydride detection tape comprising a support, composed mainly of cellulosic material, coated with an adsorbent and a solution comprising: (1) silver nitrate present in an amount of 0.5 to 2.0% by weight; (2) a polyvalent alcohol humectant, such as ethylene glycol, propylene glycol, trimethylene glycol, glycerine, or mixtures thereof, present in an amount of 5 to 30% by weight; (3) p-toluenesulfonic acid, present in amount of 0.5 to 3.0% by weight; and (4) an organic solvent, such as ethanol, methanol, or acetone.
However, it has been found that when p-toluenesulfonic acid is used in place of nitric acid the resulting hydride detection tape still weakens more quickly than desired. Additionally, the resulting detection tape exhibits a sensitivity to silane and germane that is undesirably low.